Conventional high frequency attenuators called “n-type high frequency attenuators” will be described with reference to FIGS. 12A, 12B, and 13.
FIG. 12A is a top view of a high frequency attenuator, and FIG. 12B is a perspective view of the high frequency attenuator. A high frequency attenuator 100 includes a dielectric base 51 made of a dielectric material, strip conductors 52a, 52b provided on a front surface of the dielectric base 51, a series resistor 54 and parallel resistors 55a, 55b provided on the front surface of the dielectric base 51, and a ground conductor 56 provided on a back surface of the dielectric base. The dielectric base 51, strip conductors 52a, 52b, and ground conductor 56 form a microstrip line. The series resistor 54 is formed in a gap 57 between the strip conductors 52a, 52b. The parallel resistors 55a, 55b are connected to the ground conductor 56 by using connection means such as a through-hole 58.
In an example of the n-type high frequency attenuators where the high frequency transmission line has a characteristic impedance of 50Ω, in order to attenuate the strength of the high frequency energy at a given frequency by 1 dB, the resistance value of the series resistor 54 is about 5.8Ω at the given frequency, and the resistance values of the parallel resistors 55a, 55b are about 870Ω.
A high frequency attenuator realizing the above resistances has physical dimensions as follows. It is assumed that the dielectric base 51 is made of alumina with a relative permittivity of 10 and has a thickness of 0.381 mm. Furthermore, it is assumed that conductors provided on the front and back surfaces of the dielectric base 51 are gold, and that the resistors have a sheet resistance of 50 Ω/square. As a result of calculation, the line width of the high frequency transmission line is about 0.36 mm. When the width of the series resistor 54 is about 0.36 mm which is the same as the width of the high frequency transmission line, a length 57 of the series resistor 54 is about 0.042 mm. When the widths of the parallel resistors 55a, 55b are 0.05 mm, the lengths of the parallel resistors 55a, 55b are about 0.87 mm.
FIG. 13 is a top view of another high frequency attenuator, in which the parallel resistors 55a, 55b of FIGS. 12A and 12B are omitted. When the attenuation of the high frequency attenuator is especially small, the resistance values of the parallel resistors 55a, 55b are extremely high, as high as about 870Ω as described above, and the existence of the parallel resistors 55a, 55b has a comparatively small effect. In such a case, even if the parallel resistors 55a, 55b are omitted, the characteristic impedance of the entire high frequency attenuator 101 is not much different from the characteristic impedance 50Ω of the strip conductors 52a, 52b, and the reflective characteristic represented by V. S. W. R. (voltage standing wave ratio) is little degraded. The high frequency attenuator 101 can be therefore constituted without the parallel resistors 55a, 55b. 
In addition to the n-type high frequency attenuators, T-type high frequency attenuators including resistors arranged in the T shape are known. In the attenuator including resistors arranged in the T shape as well, the resistance values of two series resistors connected in series and a parallel resistor are obtained by calculation so that the characteristic impedance of the two series resistors seen from the input end and the characteristic impedance value seen from the output end are equal to 50Ω. The width and length of each resistor are calculated based on the calculated resistance values of the three resistors.
The attenuator is manufactured by patterning conductor films and resistor films formed on the base by using the photolithography method, for example, on the basis of the width of the strip conductor and the width and length of each resistor, which are determined by the above calculation. Moreover, a high frequency attenuator is used in a high frequency device together with an amplifier, a frequency converter, and the like. In such a high frequency device, the attenuator attenuates the strength of the high frequency energy applied thereto.
High frequency attenuators configured to attenuate high frequency energy by several dB or more are widely used. Manufacturing the high frequency attenuators having a characteristic impedance of 50Ω and an attenuation of not less than several dB does not have any technical difficulties.
The high frequency attenuators have been known (JP, P2000-183609A, JP, UH03-44305A, for example). Moreover, there is a known micro-wave transmission line in which circuit elements are provided on a portion of the surface of the dielectric substrate exposed by forming an opening in a strip conductor (JP, PH09-270609A). JP, PH09-270609A discloses that the circuit elements are constituted of a ground pattern connected to a ground conductor on the back surface of the dielectric substrate and a thin-film resistor connected to between the ground pattern and strip conductor.
In order to finely adjust the high frequency performances including output power and gain, attenuators are required to be configured to attenuate energy by a minute amount of less than 1 dB. For example, in the case of manufacturing an attenuator with an attenuation of 0.5 dB, a series resistor 54 and parallel resistors 55a, 55b having desired widths cannot be produced. The strip conductors or gap between the strip conductors produced using photolithography, for example, vary in dimensions.
Consideration is made on the case of implementing a n-type high frequency attenuator with an attenuation of 0.5 dB. The resistance values of the series resistor 54 and the parallel resistors 55a, 55b are obtained by circuitry calculation as about 2.9Ω and about 1738Ω, respectively. Next, physical dimensions of the attenuator implementing the aforementioned resistance values are calculated. When the width of the series resistor 54 is about 0.36 mm, a pattern gap 57 between the strip conductors 52a, 52b, or the length of the series resistor 54, is about 0.021 mm. When the widths of the parallel resistors 55a, 55b are 0.05 mm, both of the lengths of the parallel resistors 55a, 55b are about 1.74 mm. The length of the pattern gap 57, namely, the length of the series resistor 54 is nearly marginal for stable patterning by the photolithography method, for example, in a manufacturing process and falls in a range where process defects are more likely to occur. Moreover, the accuracy of the patterns or gap between the patterns causes degradation of the accuracy of the resistance values, thus resulting in degraded accuracy of the attenuation of the high frequency attenuator.
Furthermore, the width of the high frequency transmission line of a high frequency attenuator using a thin dielectric base is smaller than the width of the high frequency transmission line of a high frequency attenuator using a thick dielectric base when the attenuators with the thin and thick dielectric bases have the same characteristic impedance. In other words, the thinner the base is, the narrower the width of the high frequency transmission line is for the same characteristic impedance. Accordingly, implementation of high frequency attenuators is almost impossible. In manufacturing high frequency attenuators configured to attenuate the high frequency energy by a minute amount, stable production of patterns with desired widths is marginal, thus increasing the possibility of process defects. Moreover, the low accuracy of the resistance values leads to low accuracy of the attenuation. Accordingly, the high frequency attenuators having desired attenuations are difficult to manufacture, or practically cannot be manufactured.